It is now common practice to explore the oceans of the earth for deposits of oil, gas and other valuable minerals by seismic techniques in which an exploration vessel imparts an acoustic wave into the water, typically by use of a compressed air "gun". The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels back upwardly where it is detected by microphone or "hydrophone" elements in a streamer or array towed by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed acoustic array typically comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof sheath and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the towed array is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. Most typically, this is done by constructing the hydrophone of a piezoelectric material, such as lead zirconate titanate ("PZT") and a means by which to amplify pressure variations to obtain the strongest possible signal (often by one or more diaphragms acting as tympanic collectors). The hydrophone elements are typically provided with leads or contacts to which to join electrical conductors, the electrical conductors carrying signals from the hydrophone elements to the recording equipment.
A typical towed array is taught in U.S. Pat. No. 4,160,229, which issued on Jul. 3, 1979, directed to a hydrophone streamer apparatus embodying concentric tube construction for achieving improved low noise operation. A plurality of hydrophone elements are supported within a compliant inner tube at spaced intervals therealong by rather complicated compliant mounting means. The inner tube is supported within an elongated outer jacket by compliant support means between the outer surface of the inner tube and the inner surface of the jacket. Suitable support means may comprise a plurality of trilobate devices each formed of three tubular sections equally spaced around the inner tube, the trilobate devices being located along the inner tube at positions between adjacent transducer elements.
The signals that hydrophones produce are of extremely low level. This is because the pressure signals that impinge on the hydrophones are weak, the hydrophones themselves are high impedance devices and the volume of piezoelectric material in hydrophones is minimized for cost reasons. Thus, it is very important to limit unwanted noise to preserve the faint signals.
Unfortunately, during operation, hydrophones encounter acoustic noise produced by a wide variety of sources emanating from the surrounding ocean, such as surface ocean waves striking the streamer or its towing vessel, propeller noise or swell noise from the towing vessel or even volcanos. Moreover, the towing cables leading from the vessel may strum as they are dragged through the water. The noise these sources produce lies mostly in the range below 10 Hz, increasing dramatically as the frequency approaches 0 Hz. The valid acoustic signals reflected back from the ocean floor tend to lie in a range from a few Hz to several hundred Hz.
In an effort to make the most use of available bandwidth of the data buses and to improve the hydrophone signal to noise ratio, it therefore becomes highly advantageous to filter out the noise. This frees the buses of the burden of carrying data pertaining to the noise, allowing that bandwidth to be spent instead on a higher resolution of the data pertaining to the remaining higher frequencies.
One of the ways to provide such filtering is to isolate the streamer from the towing vessel. Any structure-borne noise that the towing vessel generates (by its propeller or swell) is thus attenuated before it reaches the streamer. This isolation has been done by inserting a vibration isolation module ("VIM") at a forward end of the streamer (and also at the aft end, if a terminating buoy or rope drogue is used).
The most basic type of VIM is a loss type and employs one or more elastic ropes, acting as low-pass filters. The ropes allow constant towing forces to be transmitted to the streamer, while intermittent-energy vibrations are attenuated therein, dissipated as heat energy in the ropes. Another type of VIM is a stop band type and employs structures having different vibration propagation velocities and interfaces that create reflections, causing superpositions at selected frequencies that damp those frequencies. Stop band VIMs are relatively expensive and are limited in their ability to provide broadband filtering. Thus, for most applications, loss type VIMs are preferred.
The earliest loss type VIM employed a single length of lossy rope (a "primary rope") to attenuate vibration. The lossy rope was either of a natural fiber in the earliest embodiments or a manmade elastomer or polymer in more recent embodiments. While this was suitable for the purpose of attenuating vibration, towing force transients (such as those resulting as the towing vessel pitches in rough seas) occasionally caused the single lossy rope to stretch past the point at which it can return to its original length and flexibility. Over time, this altered the lossy rope's damping characteristics, decreasing the effectiveness of the VIM.
One step toward solving this problem was to add a second lossy rope (a "secondary rope") that only came into play when the towing force exceeded a first limit. The secondary rope introduced more resistance to stretching and, hence, changed the response of the VIM to vibration. Unfortunately, extreme towing force transients still distended both the primary and secondary ropes, forever changing their ability to filter out vibrations.
The most recent step toward solving this problem has been to provide a third rope (a "stopper rope") in the VIM. However, this rope differs from the primary and secondary ropes in that the stopper rope is extremely strong and relatively nonextensible. The function of the stopper rope is to carry towing force transients that would otherwise distend the primary and secondary ropes. Because the stopper rope is relatively nonextensible, it is not lossy and vibrations pass through to the module. However, the stopper rope is not designed to carry forces under normal operation of the streamer, and that it is better to vibrate the streamer for a short time rather than to harm the VIM long term.
These three-rope lossy VIMs have been implemented in deepwater streamers having a diameter of at least 2.8 inches by providing three rope loops, pulleys at either end of the VIM receiving the rope loops and transferring towing forces and vibration into the rope loops as desired. Since the pulleys acted to transition energy into the ropes, they are called "transitions." The pulleys were of a conventional side-by-side design, existing as a block.
As mentioned, deepwater seismic streamers have had a diameter of at least 2.8 inches, although some small streamers of limited capability and employed for specialized work have been of less diameter. This large diameter was necessary to house larger, stronger strain cables and larger diameter hydrophones. This larger diameter posed a storage problem, as such streamers are typically more than 3 km long. The sheer volume of the streamer and handling equipment exacerbated the modern practice of towing multiple streamers in an array. Further, as damaged modules must be shipped to repair sites, the larger diameter posed a shipping problem.
It has thus become very advantageous to provide a thinner streamer (of only 2 inches in diameter, for example). Unfortunately, the prior art scheme employing side-by-side pulleys in a three-rope VIM cannot be reduced to the desired 2 inch maximum diameter. What is needed in the art is a three-rope lossy VIM having narrow, more volume-efficient transitions therein.